Cracks and particular intergranular stress corrosion cracking is a major problem in stainless steel piping systems carrying hazardous fluids particularly systems containing boiling water for nuclear reactor plants. The intergranular stress corrosion cracking most frequently occurs in regions adjacent to weld areas in the pipe section in which the welding of the pipe joints engenders thermostress in the piping and adjacent the weld. The intergranular stress corrosion is generally thought to originate on the interior walls of the pipe near the weldment and then grow both radially and circumferentially into the pipe section due to the combined affects of stress, heat, and corrosion from the boiling water passing through the pipe. The intergranular stress corrosion cracks are generally referred to as "tight" cracks which visually appear as fissures or veins extending throughout a volume of the pipe.
Not only are the "tight" cracks difficult to detect but it is even more difficult to determine the size or depth of the crack into the pipe section.
With respect to nuclear power plants, a conservative replacement approach is usually taken because of the risks involved in leakage from the power plant piping system. Large stainless steel pipe sections that are welded together are generally used as principal conduits for the primary reactor coolant system. If a crack is detected, the conservative replacement criteria may mandate unplanned shut-down of the reactor. An unplanned down time of approximately a week or ten days to replace a suspected pipe section and restart the reactor is not unusual. Sometimes it is found after the fact that the detected crack was not "critical" and that the power plant could have continued operating safely until the next "scheduled" maintenance shut-down.
The problem and a proposed solution is described in the Melvin E. Lapides U.S. Pat. No. 4,393,711 granted July 19, 1983.
Accordingly, it is the principal object of this invention to provide a unique system for determining the depth or the extent of a detected crack particularly in pipe sections so that one can evaluate the seriousness of the problem. These and other objects, features and advantages of this invention will become apparent upon consideration of the following detailed description taken in conjunction with the accompanying drawings.